Automatically adjusting photoelectric position detector

ABSTRACT

A photoelectric position detector for a line or edge target provides an output of the same polarity when the target deviates in a given direction from the reference position regardless of whether the target is a left edge or a right edge or a line and irrespective of the orientation of the high reflectivity and low reflectivity portions of the target. Optic fibers transmit light reflected from discrete areas of the target to a plurality of detector photocells, means responsive to the output of at least two photocells derive a contrast signal proportional to the relative intensity of light reflected from different areas of the target, and an automatic gain control amplifier responsive to the contrast signal supplies voltage to the detector photocells and continuously adjusts its gain to a constant value so that the response of the photocells for a given target deviation is substantially constant regardless of target contrast.

United States Patent [1 1 Vi schulis 1 Feb. 27, 1973 AUTOMATICALLY ADJUSTING PHOTOELECTRIC POSITION DETECTOR [76] Inventor: George Vischulis, W172 N9409 Shady Lane, Menomonee Falls, Wis. 1

[22] Filed: Oct. 29, 1971 [21] App1.No.: 193,822

[52] US. Cl. ..250/202, 250/209, 318/577 [51] Int. Cl. ..G06k 11/02 [58] Field of Search ..250/202; 318/577; 250/209 3,198,949 Holdo ..2S0/202 Primary ExaminerJames W. Lawrence Assistant ExaminerT. N. Grigsby Attorney.lames E. Nilles [57] ABSTRACT A photoelectric position detector for a line or edge target provides an output of the same polarity when the target deviates in a given direction from the reference position regardless of whether the target is a left edge or a right edge or a line and irrespective of the orientation of the high reflectivity and low reflectivity portions of the target. Optic fibers transmit light reflected from discrete areas of the target to a plurality of detector photocells, means responsive to the output of at least two photocells derive a contrast signal proportional to the relative intensity of light reflected from different areas of the target, and an automatic gain control amplifier responsive to the contrast signal supplies voltage to the detector photocells and continuously adjusts its gain to a constant value so that the response of the photocells for a given target deviation is substantially constant regardless of target contrast.

43 Claims, 34 Drawing Figures I ii if PATENTEBQFEBZTIHYS SHEET 10F 8 FIG.2

I PATENTEDFEBZHBH R 3, 7 18.821 SHEET 2 BF 8 RECEIVER FIBER TERMINATION PATTERN K J;

TO PI-IoTo CELLS LIGHT FEEDBACK FIBERS RFc RFB 4 I09 I I I I O7 Hx FIBER TERMINATION PATTERN AT PLANE FACING TARGET TARGET ILLUMINATION FIBER TERMINATION PATTERN m 59 RELATIVE oIREcTIoN TARGET oF CENTERLINE oF LJNE oR EDGE FIBERS TO BE DETECTED FIG. 5

FIG. 4

IGHT souRcE.

DIFFusER AND HEAT ABsoRBER-I I- 3 coNDENsER LENS 45 I09 LIGHT FEEDBACK FIBERG 7 FIBER TERMINATION PLA E UGHT ADJUSTING MEANS LIGI-IT souRcE FFA FFB PFC FFD FFE H5 H5 I06 PHoTocELLG- I06 I0 FIBERS CONDUCTING LIGHTV RFA RFB -RFC RFD To THE TARGET AREA LIGHTVADJUSTING MEANS RECEIVER FIBERS FIG. 3 v FIBER TERMINATION PLANE II SENSING PLANE TARGET PLANE suRFAcE FLA'TORCURVED PAI'EI-IIEUFEBZYIQH- 3. T1 8 8 2 l SHEET 3 OF 8 WIDTH 0F DETECTOR OPERATIONAL FIELD A= VIEW AREA OF PHOTOCELL PCA C D B B= VIEW AREA OF PHOTOCELL PCB (2: VIEW AREA or: PHOTOCELL. PCC D: VIEW AREA OF PHOTOCELL PCD DIRECTION OF TARGET POSITION VARIATIONS TO BE DETECTED FIG. 6 V i a W% A c D B A c D B A c D B W LINE DARK LINE LIGHT EDGE EDGE BACKGROUND LIGHT BACKGROUND DARK DARK SIDE LEFT DARK SIDE RIGHT CONTRAST=AB CD CONTRPST=AB CD CONTRAST: A B CONTRAST A B POSITION= C? D POSITION .C D POSITION =AB=CD POSITION AB=CD FIG. 7 FIG. 7B FIG.7 C FIG. 7D

AcD cDB VACDB AcDB CONTRAST= CONTRAST= CONTRAST=- CONTRAST= AB CD AB CD r A s posmom: I osmom: POSITION= B A c D B c D CONTRAST: I CONTRAST: CONTRAST:

POSITION POSITION= R smon:

c D c D AB CD FIG 9A FIDDB FIGIQC PATENTEUFEBZYIUYS SHEET l UF 8 A w an PATENm rmamn SHEET 5 BF 3 FIG.13

FIG. 15 E0: 2

us wa I07 n5 4 //FFC 7 H4 IV AUTOMATICALLY ADJUSTING PHOTOELECTRIC POSITION DETECTOR This invention relates to apparatus for detecting the position of a line or edge target such as may be formed by printed areas of different reflectivity and for deriving an electrical output signal which is a function of the deviation of the target from a reference position.

BACKGROUND OF THE INVENTION Apparatus for monitoring an edge or line target formed by printed areas of different reflectivity is known for such purposes as photoelectric control of a longitudinally transported web, but such prior art detecting devices have not been entirely satisfactory. Should there be any change in the intensity of the light received by a photocell detector other than that resulting from deviation of the target from the centerline reference position, the lateral position of the target will be adjusted by the control system in an effort to correct the light variation. After such movement, the position of the target will vary from the reference position and will be in error. Known edge or line target position detectors do not adequately compensate for variations in light intensity encountered in use resulting from changes in contrast, reflectivity or transmissibility, width of the target, or ambient light conditions. Known line or edge target sensing apparatus is incapable of monitoring a target in which wide variations in reflectivity occur or on which widely different patterns are printed because the resulting wide variances in gain, which are unrelated to target position, cause instability of the closed loop control system.

The polarity of the output of known edge or line target position detecting devices changes when the orientation of the low reflectivity and high reflectivity portions of the target reverse, and prior art position detection apparatus required manual selection of output polarity when confronted with such target characteristic changes. For example, known edge or line target detecting devices require manually operated means to switch the polarity of the output signal in order to change from monitoring a left edge target to monitoring a right edge target, and consequently such apparatus is uncapable of monitoring a checkerboard pattern similar to that shown in FIG. 19.

Prior art position detecting apparatus cannot satisfactorily monitor an intermittent pattern similar to that illustrated in FIG. and tends to move the target laterally when intermittent spacing occurs in the target. Further, known position detecting devices require mechanical switch means to inactivate the apparatus when no target is present in order to prevent erroneous lateral movement of the target. Also, known position detection devices do not adequately compensate for the differences in the light intensity vs. resistance characteristics of the individual detector photocells.

SUMMARY OF THE INVENTION The invention provides an automatic adjusting photoelectric position detector which obviates the above disadvantages of prior art apparatus, and which is fail-safe and can monitor an intermittent target such as illustrated in FIG. 20 without erroneous lateral movement of the target. Another aspect of the invention provides an automatic adjusting photoelectric position detector which can monitor a checkerboard pattern such as illustrated in FIG. 19 and does not require manual switching means to change between monitoring a left edge target and a right edge target.

The invention furthermore provides an automatic adjusting photoelectric position detector wherein the output always has the same polarity when the target is laterally displaced in a given direction from a reference position regardless of whether the target is a line or a right edge or a left edge and irrespective of the orientation of the low reflectivity and high reflectivity portions of the target.

Another aspect of the invention provides an improved device for detecting the position of a linear edge target which automatically compensates for differences in ambient light conditions over the detection field and for differences in contrast, reflectivity or transmissibility in the target or in target width which are unrelated to target position.

Another object of the invention is to provide an automatic adjusting position detector which has constant gain irrespective of differences in patterns printed on the target, in contrast, in reflectivity or in opacity, in target width or in ambient light conditions.

A still further object of the invention is to provide an automatic adjusting position detector having a plurality of photocells monitoring the target inwhich all of the detector photocells respond in an identical manner to a given level of illumination.

These and other objects and advantages of the invention will appear hereinafter as this disclosure progresses, reference being had to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical section view through the scanning head of a preferred embodiment of the invention;

FIG. 2 is a vertical sectional view through the scanning head of FIG. 1, taken at right angles thereto;

FIG. 3 is a schematic view'of the light paths in the scanning head of FIG. 1;

FIG. 4 is a view taken along line 4-4 in FIG. 1 showing the termination pattern of the illumination and receiver optic fibers at the plane facing the target;

FIG. 5 is a view taken along line 5-5 in FIG. 1 showing the termination pattern of the illumination and feedback optic fibers at the plane facing the condenser lens;

FIG. 6 schematically illustrates the areas viewed by the four detector photocells in the scanning head of FIG. 1; 7

FIGS. 7A, 7B, 7C, and 7D show the areas viewed by the four detector photocells with a target centered at the reference position within the position detection field when the target is respectively: (a) a dark (low reflectivity or transparant) line on a relatively light (high reflectivity) background;.(b) a light (high reflectivity) line on a relatively dark (or transparant) background; (c) a right edge; and (d) a left edge;

FIGS. 8A, 8B, 8C and 8D are views similar to FIGS. 7A, 7B, 7C and 7D, respectively, but with the target to the left of the centerline reference position;

FIGS. 9A, 9B, 9C and 9D are views similar to FIGS. 7A, 7B, 7C and 7D, respectively but with the target to the right of the centerline reference position;

FIG. is a schematic circuit diagram in block form of a preferred embodiment of the invention;

FIG. 11 is a logic table showing the polarity and condition of elements of the circuit of FIG. 10 for each of the targets and target positions represented in FIGS. 7A7D, 8A-8D and 9A9D;

FIG. 12 is a schematic circuit diagram of a single photocell adjusting network of the circuit of FIG. 10;

FIG. 13 is a schematic circuit diagram of the automatic gain control shown in block form in FIG. 10;

FIGS. 14A and 14B are schematic circuit diagrams of power supply switches PSI-PS4 shown in block form in FIG. 10;

FIG. 15 is a schematic circuit diagram of the photocell signal summing networks AB and CD shown in block form in FIG. 10;

FIG. 16 is a schematic circuit diagram of the means for maintaining the light source lamp at a constant light output;

FIG. 17 is a view taken along line 17-17 of FIG. 1;

FIG. 18 is a schematic circuit diagram of the temperature control for the scanning head shown in FIGS. 1 and 2;

FIGS. 19 and 20 illustrate typical checkerboard and intermittent targets respectively;

FIGS. 21 and 22 schematically represent electrical network for an individual photocell and the network for all four detector photocells respectively; and

FIGS. 23 and 24 illustrate adjustment of the detector photocells by optical means so that each is exposed to the same effective illumination level.

DESCRIPTION OF A PREFERRED EMBODIMENT Scanning Head The scanning head of the preferred embodiment of the invention shown in FIGS. 1 and 2 detects the position of a line or edge target such as formed by printed areas of different reflectivity or by the edge of a longitudinally transported web and includes a generally box-shaped body 40 which may be supported by pairs of mounting studs 41 extending horizontally from opposed sidewalls thereof. Body 40 has an upwardly facing square recess 42 therein which encloses a condenser lens 45 and the upper end of a tubular optics conduit 46 having its axis aligned with the optical axis of lens 45. A square light source base 47 having a central aperture 48 therethrough is secured to body 40 by bolts 50 extending through clearance holes in body 40. Light source base 47 has an annular horizontal shoulder 51 around the margin of aperture 48 which supports a heat absorbing diffuser glass 53 above con denser lens 45. A cup-shaped light source housing guide 54 is secured abovebase 47 by the through bolts 50, and an inverted cup-shaped light source housing 56 fits within housing guide 54.

A light source lamp LS is removably secured within a lamp mounting socket 58 so that lamp LS is positioned above diffuser 53 and lens 45. Lamp LS preferably has a spirally wound, elongated but narrow filament 59 shown in FIGS. 1, 2 and 5. Socket 58 is affixed to a U- shaped bracket 60 having flexible legs and portions 62 and 63 extending horizontally from the legs thereof and being so oriented that the elongated dimension of filament 59 is parallel to the reference position centerline as shown schematically in FIG. 5 and described hereinafter. Portion 62 is affixed to the top wall 64 of light source housing 56 by a screw 65 extending through top wall 64 and surrounded by a tubular spacer 67 disposed between the top wall 64 and portion 62. The vertical position of horizontal portion 63 of bracket 60 may be selectively changed by a position adjusting screw 68 to move the filament 59 of lamp LS from side to side in a direction transverse to the reference position centerline and thus adjust the position of the lamp filament 59 relative to condenser lens 45. Lamp position adjusting screw 68 extends through clearance holes in top wall 64 and in horizontal portion 63 and is engaged by a nut beneath portion 63. A compression spring 70 surrounding adjusting screw 68 urges horizontal portion 63 away from top wall 64. A depending lip 71 on portion 63 engages the nut on adjusting screw 68 and prevents the nut from turning, and it will be apparent that turning of adjustingscrew 68 moves horizontal portion 63 of bracket 60 toward or away from top wall 64 to adjust the position of lamp LS. Adjustment for major changes of filament location from lamp to lamp are accomplished by lamp positioning adjusting screw 68.

A light reflector tube 75 surrounds lamp LS and fits within opening 48 in light source base 47 above the heat absorbing diffuser 53. Reflector tube 75 reflects light from filament 59 downward to the edges of diffuser 53 to provide relatively constant illumination across the detector operational field.

Condenser lens 45 is secured to a cup-shaped lens housing 74 which is supported by threaded vertical studs 76 (see FIG. 2) from the top wall 64 of light source housing 56. V

The square recess 42 in body 40 communicates with a horizontally extending circular entrance opening 76 in body 40 which receives an electrical cable. Body 40 also has a vertically extending cylindrical aperture 78 communicating with recess 42 which receives the tubular optics conduit 46. The optics conduit 46 has a central opening of generally rectangular cross section through which a bundle of target illuminating optic fibers 79 extend for illuminating the target with light from lamp LS. Bundle 79 preferably comprises a plurality of optic fibers which are suitably coated so that light is transmitted at high efficiency from one end of the bundle to the other.

A square optics plate 82 is secured to the lower surface of body 40 by the through bolts 50 and has a central aperture through which optics conduit 46 protrudes. A cup-shaped optics housing 84 having a central opening 85 therethrough for the target illuminating optic fiber bundle 79 is disposed beneath optics' plate 82. An optics block 87 which is generally T- shaped in cross section is disposed below optics housing 84, and cap screws 88 extend through clearance holes in optics block 87 and in optics housing 84 and in optics plate 82 and engage nuts (not shown) in optics plate 82 to support optics housing 84 and optics block 87 on body 40.

Optics block 87 has a central square opening 90 therethrough which receives the bundle of target illuminating optic fibers 79 that transmit light from lamp LS to illuminate the target. The termination pattern of the target illumination optic fibers 79 at the plane facing the target as shown in FIG. 4, and it will be noted that the target illuminating optic fibers 79 are intermeshed with and disposed on both sides of bundles of receiver optic fibers RFA, RFB, RFC and RFD which receive light reflected from the target and transmit it to detector photocells. A protective glass lens 91 covers the ends of the optic fibers 79 which illuminates the target and the light receiving ends of the receiver optic fiber bundles, and a lens retainer 92 affixed by screws to optics housing84 holds lens 91 in place.

A printed circuit board 94 is disposed within recess 42 in body 40 and has a central opening surrounding optics conduit 46. Printed circuit board 94 carries the four detector photocells PCA, PCB, PCC and PCD, and a light source control photocell LSC, but only one photocell, namely photocell LSC is shown in FIG. 1. The light paths through the optic fibers are shown schematically in FIG. 3. It will be noted that the optic fibers of bundle 79 which extend through optics conduit 46 illuminate the target, and individual bundles of receiver optic fibers RFA, RFB, RFC and RFD transmit light reflected from discrete areas of the target to the detector photocells PCA, PCB, PCC and PCD, respectively. Only a single bundle RFC of receiver optic fibers is illustrated in FIG. 1, but it will be noted that receiver fiber bundles RFA, RFB, RFC and RFD extend through opening 90 in optics block 87 and through optics housing 84 and transmit the reflected light to the individual detector photocells PCA, PCB, PCC and PCD. FIG. 4 illustrates the termination pattern of the receiver fiber bundles RFA, RFB, RFC and RFD at the plane facing the target. The reflected light receiving ends of the four receiver optic fiber bundles RFA, RFB, RFC and RFD are spaced apart in a direction perpendicular to the reference position centerline at the position detector field, and the reflected light receiving ends of receiver optic fiber bundles RFA, RFB, RFC and RFD are intermeshed with target illuminating optic fibers so that target illuminating optic fibers 79 are disposed on both sides of each receiver optic fiber bundle at the plane facing the target as seen in FIG. 4. Further, the ends of the receiver fiber bundles RFA and RFB which receive the light reflected from the target and transmit it to photocells PCA and PCB are disposed on opposite sides of the reference position centerline and outwardly from the ends of the receiver fiber bundles RFC and RFD which transmit light reflected from the target to photocells PCC and PCD. It will be appreciated that the optic fibers are shown greatly enlarged in the drawing and that the target illuminating'and receiver optic fiber bundles have a total cross section approximately one-quarter inch by fivesixteenths inch at the plane facing the target.

The receiver fiber bundles RFA, RFB, RFC and RFD terminate at their upper end within vertically extending apertures 101 (See FIG. 17 in optics plate 82 which register with vertically extending light transmitting apertures 102 (see FIGS. 1 and 2) in body 40 in which the detector photocells PCA, PCB, PCC and PCD and light source control photocell LSC on printed board 94 are mounted. Horizontally extending apertures 104 in body 40 register with the vertical light transmitting apertures 102 in body 40 through which the light from the receiver fiber bundles RFA, RFB, RFC and RFD is transmitted to the detector photocells PCA, PCB, PCC

and PCD, and feedback light is transmitted to photocell I LSC. Nylon inserts within apertures 104 have female threads that engage light adjusting screws 106. The screws 106 can be turned manually to a position wherein they block a portion of the light transmitted from one of the receiver fiber bundles RFA through RFD to the corresponding detector photocell PCA, PCB, PCC or PCD.

Optics conduit 46 has a keyway slot 107 in which a bundle 109 of feedback optic fibers is disposed. At its upper end the feedback fiber bundle 109 receives light from condenser lens 45. Feedback bundle 109 extends through keyway slot 107 and is divided into five feedback fiber bundles FFA, FFB, FFC, FFD and FFE. The four feedback fiber bundles FFA, FFB, FFC and FFD terminate within feedback fiber receiving apertures 111 in optics plate 82 which register with the light transmitting apertures 102 in body 40 through which the light from the receiver bundles RFA, RFB, RFC and RFD is transmitted to the detector photocells PCA, PCB, PCC and PCD. The feedback fiber bundle FFE also terminates within an aperture 111 in optics block 82 which registers with the light transmitting aperture 102 in body 40 in which the light source control photocell LSC is disposed. Horizontal apertures 113 in optics plate 82 register with vertical apertures 111, and internally threaded nylon inserts 114 within horizontal apertures 113 engage light adjusting screws 115 which may be manually turned to block a portion of the light from the feedback fibersFFA, FFB, FFC and FFD- received by photocells PCA, PCB, PCC and PCD, respectively.

Condenser lens 45 and diffuser 53 provide uniform illumination across the target illuminating fiber bundle 79 and across the feedback fiber bundle 109 so that the position of the filament 59 of lamp LS is not critical. Inasmuch as lamp filament 59 is elongated in a direction parallel to the reference position centerline and is longerin this direction than the bundles of target illuminating and feedback optic fibers as schematically shown in FIG. 5, all of the target illuminating and feedback optic fibers receive substantially uniform illumination in this direction. However, the position of the filament 59 may vary from lamp to lamp in a direction transverse to the reference position centerline, and lamp adjusting screw 68 permits changing of the position of lamp filament 59 from side-to-side along this dimension so that the optic fibers at the extreme limit of the detector operational .field receives the same amount of light as the optic fibers of the center of the field. Thus turning of lamp adjusting screw 68 to move lamp filament 59 from side-to-side in a direction perpendicular to the reference position centerline permits adjustment of the amount of light received by detector photocell A relative to that received by detector photocell B, i.e., so that the portions of the target viewed by detector photocells A and B (which are further removed from the reference position) are illuminated equally by lamp LS.

SCI-IEMATIC CIRCUIT DIAGRAM FIG. 10 is a functional diagram in block form of a preferred embodiment of the automatic adjusting photoelectric position detector of the invention which continuously monitors the position of a line or edge target within the position detection field (which Is illustrated in FIG. 6 as the width of the view areas of the photocells PCC and PCD as seen through the receiver fiber bundles RFC and RFD, respectively). The preferred embodiment of the invention is described as having reflected light receiver fiber optics for determining the particular area of the field that each detector photocell monitors, but it will be appreciated that light from the detection field can also be transmitted to the detector photocells by other means such as apertures, collimation apparatus, lenses or focusing systems, or even by disposing the photocells in the proper position relative to the target. However, the optic fibers of the preferred embodiment provides an intense, uniform illumination of the sensing field and closely control the size, shape, and location of the lightreceptive areas of the photocells and also facilitates adjustment of photocells response characteristics as described hereinafter.

FIGS. 7A, 7B, 7C and 7D show a target centered at the reference position within the position detection field when the target is respectively: (a) a low reflectivity (or transparant) line on a relatively light (high reflectivity) background,(b) a high reflectivity line on a relatively dark (or transparant) background; (c) a right edge and (d) a left edge. The direction of target position deviation to be detected is shown in FIG. 6 and is perpendicular to the reference position centerline illustrated in FIG. 4.

EDGE DETECTION MODE When the target is an edge, a two-position multi-pole selector switch 115 is manually actuated to the upper or EDGE position as shown in FIG. 10. When the target is a line, the switch 115 is manually actuated to the lower LINE position. The four detector photocells PCA, PCB, PCC and PCD are schematically shown in FIG. 10 as rectangles, designated A, B, C and D respectively, and this symbol represents both the corresponding photocell and the light adjusting circuit therefor. The electrical adjusting circuit for each detector photocell PCA, PCB, PCC and PCD is the same and is shown in detail in FIG. 12 and discussed hereinafter. The outputs of the detector photocells PCA, PCB, PCC and PCD are respectively coupled to the non-inverting input of power amplifiers l, 2, 3 and 4 of the operational amplifier type and whose voltage gains are equal. The outputs of power amplifiers l and 2 are coupled to the inputs of a two-input photocell signal summing network, or averaging network AB shown in block form which provides an output voltage that is the algebraic average of its two input voltages and the circuit of which is shown in FIG. 15. The averaging network AB provides the output voltage where E, and E are the input voltages. The output of averaging network is designated AB where AB (A+B)/2 and A and B signify signals proportional to the output voltages from the photocells PCA and PCB, respectively.

Similarly, the outputs of power amplifiers 3 and 4 are coupled to the input of a photocell signal summing network CD shown in block form whose output voltage CD is the average (C+D)/(2) of the two input voltages C and D from power amplifiers 3 and 4 which amplify the output signal from the photocells PCC and PCD, respectively.

The output AB of averaging network AB at point 22 is coupled through pole M83 of selector switch MS to the inverting input of an output preamplifier 7 of the differential amplifier type and also to the non-inverting input of an output preamplifier 8 of the differential amplifier type. The output of the averaging network CD at point 23 is coupled through pole MS4 of selector switch MS to the non-inverting input of output preamplifier 7 and also to the inverting input of output preamplifier 8.

The power supply to preamplifier 7 is controlled by voltage actuated power supply switches PS1 and PS2, and the power supply to preamplifier 8 is controlled by similar voltage-actuated power supply switches PS3 and PS4. The circuit diagrams of the voltage actuated power supply switches PS1 through PS4 are shown in FIGS. 14A and 14B.

Preamplifiers 7 and 8 share common signal sources (AB at point 22 and CD at point 23) for their inputs, but the signal sources are fed to opposite inputs of preamplifiers 7 and 8 so their outputs will be numerically equal in voltage but opposite in polarity. When the power supply to preamplifiers 7 and 8 is cut off, these preamplifiers are in the non-conductive condition and becomes passive circuit elements of very high im pedance which effectively isolate their output terminals from their input terminals. The output terminals of preamplifiers 7 and 8 are commoned at point 24 and connected: (1) to the input of an output power amplifier 9; and (2) to the junction of two fail-safe level adjusting resistances F R] and F R2, one of which can be a potentiometer. The opposite ends of fail-safe resistances FRI and FR2 are connected to the positive and negative terminals of the power supply, and it will be appreciated that the fail-safe resistances FRI and FR2 ground the input 24 of output amplifier 9. Output power amplifier 9 amplifies the output of either preamplifier 7 or 8 depending upon which one is active and supplies the output signal of desired magnitude and polarity to suitable means (not shown) for returning the target to the reference position within the position detector field. Such target positioning means may neutralize at ground potential, but it will be appreciated that the fail-safe signal can be ground potential or any positive or negative voltage selected.

The output of photocell PCA after being amplified in power amplifier 1 is coupled through pole MSl of selector switch MS to the inverting input of a contrast differential amplifier 5 and also to the non-inverting input of a contrast differential amplifier 6. The output of photocell PCB after being amplified by power amplifier 2 is coupled through pole MS2 of selector switch MS to the non-inverting input of differential contrast amplifier 5 and the inverting input of differential contrast amplifier 6. The input terminals of differential amplifiers 5 and 6 are thus connected to the common signal sources (signal A at point 11 and signal B at point 12) in reverse to each other so that the output of differential amplifier 5 is numerically equal to opposite in polarity to the output of differential amplifier 6.

The output of contrast differential amplifier 5 at point 13 is coupled to the input of power supply switches PS2 and PS3, and the output of differential amplifier 6 at point 14 is coupled to the input of power supply switches PS1 and PS4. Power supply switches PS1 through PS4 are voltage-actuated transistor-resistor logic (TRL) switches whose circuits are shown in FIGS. 14A and 14B and which control the availability of the power supply to output preamplifiers 7 and 8. Power supply switch PS1 only turns on to connect the positive power supply to preamplifier 7 when the output of differential amplifier 6 at point 14 reaches a predetermined negative potential, e.g., 3.0 volts, and similarly power supply switch PS2 only turns on to connect the negative power supply to preamplifier 7 when the output of differential amplifier 5 (at point 13) reaches a predetermined positive potential, e.g., +3.0 volts. Power supply switch PS 3 only turns on to connect the positive power supply to output preamplifier 8 when the output of differential amplifier 5 at point 13 reaches a predetermined negative potential, and similarly power supply switch PS4 only turns on to connect the negative power supply to output preamplifier 8 when the output of differential amplifier 6 (at point 14) attains a predetermined position potential, e.g., +3.0 volts. Consequently, if the outputs of differential amplifiers 5 and 6 are +3.0 and 3.0 volts respectively, power supply switches PS1 and PS2 will both be on and preamplifier 7 will be active. Similarly, if the outputs of differential amplifiers 5 and 6 are 3. and +3.0 volts respectively, power supply switches PS3 and PS4 will be turned on and preamplifier 8 will be active. In-

asmuch as the outputs of contrast differential amplifiers and 6 are always opposite in polarity, only the pair of switches PS1 and PS2 or the pair of switches PS3 and PS4 can be on at a given time, and consequently only output preamplifier 7 or preamplifier 8 can be active at any one time.

Target positions to the right of the reference position centerline always produce a negative output relative to neutral (ground) from output power amplifier 9 at point 25, whereas target positions to the left of the reference position centerline always produce a positive output relative to ground from power output amplifier 9 at point 25, and this is true regardless of whether the target is a line or a right edge or a left edge and irrespective of the orientation of the low reflectivity and high reflectivity portions of the target. Stated in another way, the output sense remains the same regardless of whether the target is a left edge or a right edge or a line and irrespective of the polarity of contrast of the target.

When selector switch MS is in the EDGE position (edge detection mode), the contrast (target) polarity is determined by comparing signals A and B from photocells PCA and PCB. If the target is a right edge as illustrated in FIGS. 7C (edge centered), 8C (edge to left of center), and 9C (edge to right of center), photocell PCA receives less light than photocell B; A B; the output of differential amplifier 5 is positive; the output of differential amplifier 6 is negative; power switches PS1 and PS2 are on so preamplifier 7 is active; and power switches PS3 and PS4 are off so preamplifier 8 is off. The magnitude of the detector output signal from amplifier 9 at point is determined by the numerical difference of signals AB and CD, and the polarity of such position detector output signal from amplifier 9 is determined by the relative polarity of signals AB and CD and which of the preamplifiers 7 or 8 is active. It will be appreciated that a signal such as AB becomes more positive with increasing light to the corresponding photocells. Still assuming that the target is a right edge, preamplifier 7 will be active but the output of amplifier 9 will be zero when the edge is ceritered as illustrated in FIG. 7C so that AB equals CD.

Preamplifier 7 will be active but the output of amplifier 9 will be positive when the right edge target is left of the reference position as shown in FIG. 8C so that AB CD and the more positive signal CD is applied to the noninverting input of preamplifier 7. The polarity of the position detector output signal from amplifier 9 is negative when the right edge target deviates to the right of the reference position as shown in FIG. 9C so that AB CD and the more positive signal is applied to the inverting input of preamplifier 7. The magnitude of the target position detector output signal from output amplifier 9 at point 25 is dependent upon the numerical difference of signals AB and CD.

Recalling that in the edge detection mode the contrast (target) polarity is determined by comparing signals A and B from photocells PCA and PCB, if the target is a left edge as shown in FIGS. 7D (edge centered), 8D (edge to left of center) and 9D (edge to right of center), photocell PCA receives more light than photocell PCB; A B; the output of contrast different amplifier 6 is positive and that of contrast differential amplifier 5 is negative; power switches PS1 and PS2 are off so preamplifier 7 is off, and power switches PS3 and PS4 are on so preamplifier 8 is active. The polarity of the position detector output signal at point 25 is determined by the relative polarity of signals ABand CD. With the left edge target centered as shown in FIG. 7D, AB equals CD and the output from active preamplifier 8 is zero. With the left edge target to the left of the reference position centerline as shown in FIG. 8D, AB CD and the output of amplifier 9 is positive because the more positive signal AB is coupled to the non-inverting input of active preamplifier 8. With the left edge target to the right of the reference position centerline as shown in FIG. 9D, AB CD and the output of amplifier 9 is negative because the more positive signal CD is coupled to the inverting input of active output preamplifier 8.

As described hereinbefore, the power switches PS1 through PS4 do not turn on until the contrast is sufficiently great so that the outputs of contrast differential amplifiers 5 and 6 of the required polarity reach a predetermined magnitude. In other words if the target contrast is too small for the detector to operate satisfactory, the contrast polarity switches PS1 through PS4 will remain off and the output of amplifier 9 will remain at a predetermined fail-safe value. Thus, when no-valid target is in the position detection field, the 'output signal from output amplifier 9 does not change. The position detection apparatus thus reverts to a fail-safe output which may represent neutral, or inaction by the means (not shown) for positioning the target laterally.

Prior art position detectors cannot monitor intermittent patterns such as shown in FIG. 20 and tend to move the target laterally when anintermittent spacing occurs in the target. Such prior art position sensing devices require mechanical switching means to inactivate the apparatus when no target is present in order to prevent false outputs and erroneous lateral movement of the target. Prior art position detection apparatus requires manually operated means to switch the polarity of the output signal in order to change from monitoring a right edge to monitoring a left edge, and consequently such prior art position detection apparatus cannot monitor the edges formed by the squares of a checkerboard pattern, such as illustrated in FIG. 19. The position detector disclosed herein automatically changes the contrast polarity at the outputs of differential amplifiers and 6 when changing between monitoring a left edge and a right edge target and thus is capable of automatically monitoring a checkerboard pattern such as illustrated in FIG. 19 without the manual switching means required by the prior art apparatus.

LINE POSITION DETECTION Selector switch MS is manually actuated to the lower, or LINE position when the target is a line. In the LINE position of selector switch MS, the output of averaging network CD is coupled through pole MSIto the inverting input of contrast differential amplifier 5 and also to the non-inverting input of contrast differential amplifier 6. Similarly, the output of averaging network AB is coupled through pole MS2 to-the noninverting input of differential amplifier 5 and to the inverting input of differential amplifier 6. The output of photocell PCC after being amplifier by power amplifier 3 is coupled through pole MS3 to the inverting input of preamplifier 7 and to the non-inverting input of preamplifier 8. The output of photocell PCD after being amplified in power amplifier 4 is coupled through pole M84 to the non-inverting input of preamplifier 7 and to the inverting input of preamplifier 8.

In the line position detection mode, contrast (target) polarity is determined by comparing signals AB and CD, and the magnitude of the position detection output signal from the amplifier. 9 at point 25 is determined by the numerical difference of the output signals from photocells PCC and PCD, as amplified by preamplifier 7 or 8, whichever is active. The polarity of the target position signal at the output of amplifier 9 is determined by the relative polarity of the signals from photocells PCC and PCD and which of the output preamplifiers 7 or 8 is' active.

Assume the target is a low reflectivity line against a relatively high reflectivity background as shown in FIGS. 7A (line centered), 8A(target to left of center) and 9A (target to right of center). With this condition; AB CD; the input AB to the non-inverting input to contrast differential amplifier 5 is greater than the signal CD to its inverting input so the output thereof is positive; the output of contrast differential amplifier 6 is negative; contrast position switches PSI and PS2 are on so that preamplifier 7 is active; and contrast position switches PS3 and PS4 are off so that preamplifier 8 is off. When the line target is centered as shown in FIG. 7A, the outputs from photocells PCC and PCD are equal, and the target position signal amplified by active preamplifier 7 appearing at the output of amplifier 9 is zero. When the low reflectivity line target is to the left of the reference position centerline as shown in FIG.

8A, the output signal from cell PCC is less than the output signal from cell PCD; the magnitude of the position detector output signal appearing at the output of amplifier 9 is determined by the numerical difference of signals C AND D; and the output from amplifier 9 at point 25 is positive since the signal D applied to the non-inverting input of active preamplifier 7 is greater than the signal C to its inverting input. When the low reflectivity line target is to the right of the reference position centerline as shown in FIG. 9A, the signal from photocell PCC the output signal from photocell PCD; the position detector output signal appearing at the output from amplifier 9 at point 25 is determined by the difference of signals C and D and the polarity of this signal is negative since the input signal C to the inverting input of active preamplifier 7 is greater than the input signal D to the non-inverting input thereof.

If the target is a high reflectivity line against a relatively low reflectively background as shown in FIGS. 78, 8B and 9B, the light received by photocells PCC and PCD is greater than the light received by photocells PCA and PCB; signal AB signal CD; the output of contrast differential amplifier S is negative since the signal CD to its inverting input is greater than the signal AB applied to its non-inverting input; the output of contrast differential amplifier 6 is positive; power supply switches PS1 and PS2 are off so that preamplifier 7 is off; and power supply switches PS3 and PS4 are on so that preamplifier 8 is active. If the line target is centered in the position detection field as shown in FIG. 73, equal amounts of light are received by photocells PCC and PCD and the output of active preamplifier 8 and output amplifier 9 is zero. If the high reflectivity line target is to the left of the reference position centerline as shown in FIG. 8B, the output signal from photocell PCC the output signal from the photocell PCD so that the polarity of the output from active preamplifier 8 and the position detector output signal appearing at the output of amplifier 9 is positive. If the high reflectivity line target is to the right of the reference position centerline as shown in FIG. 9B, photocell PCD receives more light than photocell PCC; signal C signal D, and the polarity of the position detector output signal appearing at the output of amplifier 9 is negative.

The logic table of FIG. 11 summarizes all of the possibletarget conditions illustrated in FIGS. 7, 8 and 9 and illustrates that the output sense remains the same irrespective of whether the target is a line or a left edge or a right edge and also irrespective of the polarity of contrast of such target. Stated in another manner, target positions to the right of the reference position centerline always produce a negative output with respect to ground from output amplifier 9 at point 25, and target positions to the left of the reference position centerline always produce a positive output with respect to ground from amplifier 9 at point 25.

Automatic Gain Control Targets of different contrast ratios (i.e., ratio of the lowest light reflectivity portion of the target to the portion of highest reflectivity) monitored by known position detection devices produce responses of different magnitudes from the detector photocells for a given deviation of the target from the reference position.

Such differences in photocell response are independent of lateral deviations of the target and therefore result in different detector outputs unless means are provided to adjust the gain of the position detector. Many known position detection devices require manual adjustment of gain to compensate for targets having different contrast ratios.

The position detector of the invention connects the sensing photocells PCA, PCB, PCC and PCD in a closed loop amplifier control system which continuously and automatically adjusts the gain of the position detector to a constant value regardless of variations in contrast ratio of'the target. This is accomplished by an automatic gain control amplifier AGC shown in block form in FIG. which provides a controlled power supply to the light adjusting circuits for the detector photocells PCA, PCB, PCC and PCD. The automatic gain control amplifier AGC in effect decreases the voltage to all four detector photocells PCA, PCB, PCC and PCD when an increase in target contrast occurs thereby providing a constant target position-to-output relationship.

In the edge detection mode, the difierence in the responses of the photocells PCA and PCB which view areas of the target further removed from the reference position centerline is utilized as an indication of contrast in the target. The contrast logic signals from photocellsPCA and PCB as amplified by power amplifiers 1 and 2, are inputs of the contrast differential amplifiers 5 and 6. The outputs of differential amplifiers 5 and 6 appearing at points 13 and 14 (which outputs are equal in magnitude but opposite in polarity) are coupled through similarly poled steering diodes D1 and D2 to the input of the automatic gain amplifier AGC. The output of that differential amplifier 5 or 6 which is positive in polarity is coupled through the corresponding steering diode D1 or D2 to the input at point 15 of automatic gain control amplifier AGC whose circuit is shown in FIG. 13.

Automatic Gain Control Amplifier AGC The output of contrast differential amplifier 5 or 6 which is positive in polarity is coupled through the corresponding steering diodes D1 or D2 to point 15 and through an input resistor R1 to the base of a high gain amplifier NPN transistor Q1. The series arrangement of a capacitor C1 and a resistance R2 is connected in shunt to resistance R1 and forms a phase lead network which increases the gain of the Q1 stage when the contrast input of point 15 is rapidly changing, thereby providing improved high frequency response. The collector of transistor Q1 is coupled through a resistance R5 to the positive terminal of a DC power supply, and the emitter of transistor Q1 is connected through a resistance R4 to the emitter of a PNP transistor Q2 whose collector is coupled to the negative terminal of the power supply. A resistor R3 connects the base of transistor O1 to the emitter of transistor Q2. The resistance R1, R3, R4, and R5 determine the DC gain of the Q1 amplifier stage. The resistances R1 and R3 are selected so that transistor Q1 begins to conduct when an input signal of approximately 4.0 volts appears at point 15. The base of transistor 02 is coupled to the junction of two resistances R7 and R6 forming a voltage divider between the negative terminal of the power current through transistor Q1 and the voltage drop across collector resistor R5 and thus lower the voltage at the collector of transistor Q1 and the positive and negative output voltages at points 16 and 17 respectively from amplifier AGC.

The collector of transistor Q1 is connected to the base of a PNP transistor Q3 having its emitter connected through a resistance R9 to the positive terminal.

of the power supply and its collector coupled through a resistance R8 to the negative power supply terminal. Resistance R8 is equal to resistance R9, and the gain of the Q3 stage is preferably equal to one. The emitter and collector of transistor Q3 are equal in voltage and of opposite polarity. The emitter of transistor O3 is coupled through an emitter follower (current amplifier) transistor Q4 to the positive output terminal of amplifier AGC at point 16, and the collector of transistor O3 is coupled through an emitter follower (current amplifier) transistor Q5 to the negative output terminal of the AGC amplifier at point 17.

The positive and negative voltages with respect to ground from amplifier AGC appearing at points 16 and 17 are impressed across the individual networks A, B, C and D for the detector photocells PCA, PCB, PCC and PCD shown in block form in FIG. 10 and in detail in FIGS. 12 and 22 each of which comprises the serial arrangement of a photocell (such as PCA), a load resistance R14 and a trimming potentiometer R13. Resistances R14 are chosen to match the resistance of the corresponding photocells at a predetermined level of illumination as described hereinafter. The output of each photocell network A, B, C and D appearing at the junction of the photocell and its load resistance R14 is applied to the input of the corresponding power amplitier 1, 2, 3 or 4.

As the positive input of amplifier AGC increases to a predetermined level as a result of increase of contrast of the target, transistor Q1 of the AGC amplifier is turned on, and amplifier AGC begins to decrease the magnitude of its positive and negative outputs at terminals l6 and 17 which are the inputs to the photocell network A, B, C and D for the photocells PCA, PCB, PCC and PCD respectively. The outputs of the photocell networks A, B, C, and D decrease in direct proportion to the decrease of the AGC amplifier outputs at points 16 and 17, and therefore the inputs at points 11 and 12 to the contrast differential amplifiers 5 and 6 also decrease until equilibrium is reached between the AGC amplifier outputs at points 16 and 17 and the inputs to the contrast differential amplifiers at points 11 and 12.

Since the gain of amplifier AGC is very high, only slight input signal change at point 15 is required to release the full output range of amplifier AGC. Once the threshold of activation of amplifier AGC is attained by turning on transistor Q1, the contrast signals at points 11 and 12 are held virtually constant in magnitude. Inasmuch as the networks A, B, C and D for all four photocells PCA, PCB, PCC and PCD have the supply voltages to the photocell adjusting networks becomes. Since contrast differential amplifier 5 or 6 in series with amplifier AGC has an extremely high loop again, the output signals from the detector photocells PCA, PCB, PCC and PCD approach a constant magnitude for a given target position regardless of target contrast. Further, since the target contrast relates directly to the response of the detector photocells, controlling the contrast signal magnitude to a constant value correspondly controls the gain of the position detector to a constant value.

Photocell Matching Adjustment Photocells PCA, PCB, PCC, PCD and LSC are preferably of the bulk-effect photoconductive type, although the arrangement of the light reception areas with respect to a target described hereinbefore is also operable with other types of photocells such as those of the photoemissive, photovoltaic, or the junction photo conductive type.

The photocell circuits shown in FIGS. 12, 21 and 22, together with the optical circuits schematically shown in FIG. 3 interconnecting the light source LS, the target and the detector photocells and the light source brilliance control shown in FIG. 16 together form a photocell matching system which makes use of both light and electrical components to cause the four detector photocells PCA, PCB, PCC and PCD to respond in an identical manner to identical illumination from the target. The biasing of the photocells through light feedback fibers FFA through FFD accomplishes a response adjustment which is not possible by adjustment of electrical parameters only.

. As described hereinbefore, the amount of light transmitted from light source LS to photocells PCA, PCB, PCC, PCD and LSC through the light feedback fibers FFA, FFB, FFC, FFD and FFE, respectively may be individually adjusted by the light adjusting screws 115 and 106 (see FIG. 3) which may be manually turned to a position wherein each obstructs a desired portion of the light transmitted from the corresponding feedback fiber bundle FFA through FFE to the associated photocells PCA, PCB, PCC, PCD and LSC.

The response of bulk-type photoconductive cells to a given level of illumination varies materially from cell to cell. The electrical resistance of such a photoconductive cell of a given active material exposed to a predetermined illumination level depends upon the physical structure of the active area, i.e., length and thickness of photoconductive material and the area exposed to light. Further, even assuming that bulk-type photoconductive cells of equal active areas could be selected, variations in transmissibility of protective coatings and windows from cell to cell make it virtually impossible to manufacture photocells that are identical in response to a given level of illumination. However,

bulk-type photoconductive cells will exhibit response characteristics in precise proportion to each other when the effective illumination on their active areas is the same. Consequently, calibration of the detector photocells PCA, PCB, PCC and PCD so that their outputs are matched under identical conditions of target reflectivity requires adjustment of the effective illumination that each photocell receives under identical target conditions.

The resistances R R R and R of the detector photocells PCA, PCB, PCC and PCD respectively may be different, but the photocells are connected in an electrical circuit schematically shown in FIG. 22 which (assuming the effective illumination of all four photocells is equal) will provide equal electrical output signals E E E and E since the response of each photocell will be in fact proportional to that of all the others. FIG. 21 schematically represents the electrical circuit for an individual detector photocell and is similar to FIG. 12 except that the photocell is represented by its resistance designated R and is shown connected in series with a single load resistor R which is equal to the sum of the trimming and load resistances R13 and R14 shown in FIG. 12.

Prior to mounting the detector photocells PCA, PCB, PCC and PCD in the scanning head, load resistors R R R and R are selected for the individual photocells PCA, PCB, PCC and PCD in a calibrator so that each load resistor precisely matches the resistance of the corresponding photocell. The calibrator may vary illumination over a controlled range and indicate response of the cell for different load resistances-so that the load resistance R for the cell under test may be selected by comparison with a master photocell and its particular load resistor.

In the electrical circuit shown in FIG. 22, the resistance of each cell (such as R and its load resistance such as R form a voltage divider across which is impressed the voltage E appearing at the output terminal 16 and 17 of the automatic gain control amplifier AGC. (The potential E equals the sum of the positive voltage +V with respect to ground appearing at terminal 16 and the negative voltage V appearing at terminal 17 The electrical output signal (such as E appears at the junction of each photo-cell (such as R with its load resistor (such as R In accordance with the laws of proportionality, all photocells so mated with load resistances and exposed to identical effective illumination will be in the relation so their voltage outputs will precisely match EOA on oc on from cell to cell. Further, the effective transmitting area of the illumination fibers and optic fibers associated with each detector photocell may be different from that for the other cells due to manufacturing tolerances. Consequently, the amount of light received by each detector photocell may be considerably different from that received by the others even though the target is identical for all detector photocells.

The light adjusting screws 106 which regulate the amount of light that the detector photocells PCA-PCD receive from the receiver optic fibers RFA-RFD and the light adjusting screws 115 that regulate the amount of light which the detector photocells PCA-PCD receive from the feedback optic fibers FFA-FFD comprise means for exposing each detector photocell PCA-PCD to the same effective illumination level. The light adjusting screws 106 for the receiving optic fibers RFA-RFD control the range (A E or amplitude of the output signals E -E as illustrated in FIG. 24, wherein the adjustment screws 115 for the feedback optic fibers FFA-FFD control the average level of the output signals B -E as illustrated in FIG. 23. Actually the two adjustments interact since raising the light level received by the detector photocells PCA-PCD with the feedback optic fiber adjustment screws 115 inherently reduces the range (change AE of the output signal or change AR of the cell resistance for a given change of illumination level A L) since the cell now operates in a lower AR area of the response curve as seen in FIG. 23. Also, increasing the amount of light received by the photocells by opening the receiver optic fiber adjustment screws 106 inherently lowers the average level of the output signals E -E FIG. 23 illustrates the effective of turning only the feedback fiber light adjusting screws 115 to increase the amount of feedback light received by the detector photocells PCA-PCD while the amount of feedback light is kept constant. The initial effective illumination level of the detector photocells is designated L the portion of the curve between points l--l plots photocell resistance R, (or electrical output signal E against effective illuminationlevel in the range L to L +(AL and it will be noted that the change in output signal A E for a given change in effective illumination level A L, 10 is equal to 10 and further that the average output level signal E, for the portion of the curve l-1 has the magnitude of l I. The portion of the curve between points 22 plots photocell resistance R, (or output signal E versus effective illumination level when the feedback fiber light adjusting screws 115 are opened to increase the initial illumination to L 10. The portion 22 of the curve covers the range of effective illumination levels from L 10 to L (A L 10), and it will be noted that the range, or change in output signal AE for the portion of the curve 22 equals only 4.5 and that the average output level for the portion 22 of the curve has the value 6.75. The complete curve shown in FIG. 23 is the resultant E (or R versus effective cell illumination L for all detector photocells PCA-PCD after the proper load resistance R R has been chosen for each cell. Since the response of each detector photocell PCA-PCD with its matched load resistance R -R must be on the resultant curve shown in FIG. 21, it will be appreciated that the twoadjustments (i.e., feedback fiber light adjustment by screws 115 and receiver fiber light adjustment by screws 116) can cause all detector photocells to have identical responses to identical levels of target reflectivity.

FIG. 24 illustrates the effect of turning the receiver fiber adjusting screws only to increase the amount of reflected light from the target transmitted to the detector photocells while the amount of feedback light is held constant. The portion l1 of the curve shows the variation in photocell resistance R (or output signal E for an initial setting L 5 of the receiver fiber adjusting screws 115 over the range L 5 to L,, +(AL =10). The change AE in output signal over the illumination range AL 10 is equal to 10, and the average output signal level for the portion of the curve l-1 has the value of ll. The portion 22 of the curve shows the photocell response when the receiver fiber adjusting screws 115 are opened to double the amount of light received through the receiver fibers to L l0 over the target illumination range L to L +(AL, 20). It will be noted that the range, or change of output signal AE equals only 6.5 even though the change in illumination level AL, is also doubled, and further that the average output level is only 5.75 for the portion 2- 2 of the curve. It will be appreciated that when the receiver fiber optic screw adjustment is changed, the initial illumination level also changes and has the same effect as the feedback fiber light adjustment except that A L is also changed, whereas in contrast, adjustment of the amount of feedback fiber light only has no effect on AL since this is dependent solely upon changes in target reflectivity.

Consequently, in order to increase the range AE, of the output signal without changing the average level of output signal, the receiver fiber adjusting screws 106 are opened and the feedback fiber adjusting screws 115 are closed (i.e., turned to reduce the amount of feedback light received by the detector photocells).

Light Source Brilliance Control The light source brilliance control shown in FIG. 16 automatically maintains the light source lamp LS at a constant light output regardless of aging effects and variation in ambient temperature, thereby keeping the detector photocells PCA, PCB, FCC and PCD in a desired range of illumination levels and permitting the photocell response characteristics to remain in precise adjustment at all times. The lamp LS is connected in series with the emitter-collector circuit of a power series transistor Q12 across the terminals of a heavy duty DC power supply. Photocell LSC receives light from lamp LS through feedback bundle FFE as shown in FIG. 3. The inverting input of an operational amplifier 30 of the light source brilliance control is connected through an input resistance R24 to a voltage divider formed by two resistances R22 and R23 connected in series between the positive terminal of a power supply and ground. The voltage divider formed by resistances R22 and R23 establishes the input bias for operational amplifier 30. Photocell LSC is connected in series with a resistance R21 between the positive terminal of the power supply and ground and forms a voltage divider for active input to amplifier 30, and the voltage at the junction of photocell LSC and resistance R21 is coupled through an input resistance R25 to the non-inverting input of amplifier 30. The output of operation amplifier 30 is coupled through a voltage regulating Zener diode Z11 to the base of a transistor Q11 which is connected in a Darlington circuit with the power transistor Q12. Zener diode Z11 in series with a resistor R27 connected between the base and the collector of transistor Q11 provides a load for amplifier 30, and the voltage drop across resistor R27 biases transistor Q11 on. A feedback resistance R26 for amplifier 30 is connected between the emitter of transistor Q1 1 and the inverting input of amplifier 30 and establishes the gain of amplifier 30 as 1+R26/R24. A capacitor C11 connected between the base of transistor Q11 and the inverting input of amplifier 30 provides degenerative feedback and keeps the light source brilliance control stabilized.

Temperature Control Body 40 is preferably constructed of aluminum, and an automatic temperature control preferably maintains body 40 at a temperature above any ambient temperature to which the position detecting head may be adjusted in operation to maintain a constant ambient temperature for the detector photocells PCA, PCB, PCC, PCD and LSC.

The circuit diagram for the temperature control is shown in FIG. 18. A resistance R31 connected in series with a Zener diode Z31 across the positive and negative terminals of a power supply form a regulated voltage supply. A thermistor RT mounted within recess 42 (not shown in FIGS. 1 and 2, so as to be in thermal heat exchange relation with body 40) is electrically connected in series with a resistance R32 across the Zener diode Z31. A PNP transistor Q31 has its emitter and base connected to opposite sides of thermistor RT and its collector coupled through a current limiting resistance R33 to the base of a power transistor switch Q32. The emitter of power transistor switch Q32 is connected to the grounded negative terminal of the power supply and its collector is connected to the positive terminal of the power supply through a resistive heater element I-l (not shown in FIGS. 1 and 2) which is mounted within recess 42 to be in thermal heat exchange relation with body 40. Body 40 thermally connects the heater element H to the thermistor RT and to the photocells PCA, PCB, PCC, PCD and LSC. Heater adds heat to body 40 so that it is maintained at a substantially constant temperature above any ambient temperature that the position detection head may encounter in operation and any changes in temperatures of body 40 are detected by thermistor RT. The resulting change in resistance of thermistor RT varies the potential across the base-emitter junction of transistor Q31, and the consequent change in collector potential of transistor Q31 is coupled to the base of power transistor switch Q32 and varies the current through resistive heater H in a direction to maintain body 40 at a substantially constant temperature.

1 claim:

1. Position detection apparatus for a line or edge target to be maintained in a reference position comprising, in combination,

a source of illumination for said target,

a plurality of detector photocells,

means for transmitting to individual ones of said photocells light from discrete areas of said target spaced apart laterally of said reference position,

means responsive to the outputs of at least two of said photocells for deriving a contrast signal whose polarity as a function of the relative intensity of light received from different areas of said target,

means responsive to the outputs of at least two of said photocells for deriving a position signal whose polarity is a function of the direction of lateral deviation of said target from said reference position, and

means responsive to the polarity of said contrast signal and of said position signal for generating an output signal of one polarity when said target deviates in one direction from said reference position and for generating an output signal of the opposite polarity when said target deviates in the opposite direction from said reference position.

2. Position detection apparatus in accordance with claim 1 and including automatic gain control means receiving said contrast signal as an input for supplying voltage to said photocells, the gain of said automatic gain control means being inversely proportional to the magnitude of the input signal thereto.

3. Position detection apparatus in accordance with claim 1 wherein said contrast signal deriving means includes first and second contrast differential amplifiers and a pair of similarly poled diodes each of which couples the output of one of said contrast differential amplifiers to the input of said automatic gain control means.

4. Position detection apparatus in accordance with claim 3 wherein the outputs of said two photocells are coupled in reverse to the inverting and non-inverting inputs of said first and second contrast differential amplifiers.

5. Position detection apparatus in accordance with claim 1 wherein said target is an edge and said contrast signal deriving means is responsive to the outputs of a first pair of said detector photocells receiving light from areas of said target spaced apart approximately equally on opposite sides of said reference position.

6. Position detection apparatus in accordance with claim 5 wherein a second pair of said detector photocells receives light from inner areas of said target on opposite sides of said reference position and said first pair of photocells receives light from outer areas of said target further removed from said reference position than said inner areas, and said position signal deriving means compares the algebraic average of the outputs of said first pair of photocells with the algebraic average of the outputs of said second pair of photocells.

7. Position detection apparatus in accordance with claim 6 wherein said light transmitting means comprises optic fiber means for transmitting light reflected from said discrete areas of said target to said detector photocells.

8. Position detection apparatus in accordance with claim 1 wherein said target is a line and said position signal deriving means is responsive to the outputs of a first pair of said detector photocells receiving light from inner areas of said target spaced approximately equal distances on opposite sides of said reference position.

9. Position detection apparatus in accordance with get on opposite sides of said reference position and spaced further therefrom than said inner areas, and said contrast signal deriving means compares the algebraic average of the outputs of said first pair of detector photocells with the algebraic average of the outputs of said second pair of detector photocells to derive said contrast signal.

10. Positiondetection apparatus in accordance with claim 9 wherein said contrast signal deriving means includes first and second averaging circuits for deriving signals which respectively are the algebraic average of the outputs of said first pair of detector photocells and the algebraic average of the outputs of said second pair of detector photocells.

11. Position detection apparatus in accordance with claim 10 wherein said light transmitting means comprises optic fiber means for transmitting light reflected from said discrete areas of said target to said detector photocells.

12. Position detection apparatus in accordance with claim 10 and including automatic gain control means receiving said contrast signal as an input for supplying voltage to said detector photocells, the gain of said automatic gain control means being inversely proportional to the magnitude of the input signal thereto.

13. Position detector apparatus in accordance with claim 12 wherein said contrast signal deriving means includes a pair of contrast differential amplifiers and a pair of similarly poled diodes coupling the outputs of said contrast differential amplifiers to the inputof said automatic gain control means, and wherein the outputs of said first and second averaging circuits are connected in reverse to the inverting and non-inverting inputs of said pair of contrast differential amplifiers.

14. Position detection apparatus in accordance with claim 13 wherein each of said detector photocells is connected in series with a resistance to form a voltage divider and said automatic gain control means generates substantially equal positive and negative voltages with respect to ground which are coupled to the ends of said voltage divider of each of said detector photocells.

15. Position detection apparatus in accordance with claim 6 wherein said position signal deriving means includes first and second averaging circuit means for deriving signals which respectively are the algebraic average of the outputs of said first pair of detector photocells and the algebraic average of the outputs of said second pair of photocells.

16. Position detection apparatus in accordance with claim 15 and including automatic gain control means receiving said contrast signal as an input for supplying voltage to said detector photocells, the gain of said automatic gain control means being inversely proportional to the magnitude of the input signal thereto.

17. Position detection apparatus in accordance with claim 16 wherein said contrast signal deriving means includes a pairof contrastdifferential amplifiers and a pair of similarly poled diodes coupling the outputs of said contrast differential amplifiers to the input of said automatic gain control means and wherein the outputs of said photocells of said first pair are coupled in reverse to the inverting and non-inverting inputs of said contrast differential amplifiers.

18. Position detection apparatus in accordance with claim 17 wherein each of said detector photocells is of the photoresistive type and is connected in series with a resistor having a resistance approximately equal to that of said photocell to form a voltage divider and said automatic gain control means generates substantially equal positive and negative voltages with respect to ground which are coupled to the ends of said voltage divider for each of said detector photocells.

19. Position detection apparatus for a line or edge target to be maintained in a reference position comprising, in combination,

a source of illumination for said target, a plurality of detector photocells, means for transmitting to individual ones of said photocells light from discrete areas of said target spaced apart laterally of said reference position,

means responsive to the outputs of at least two of said photocells for deriving a contrast signal whose magnitude is a function of the relative intensity of light received from different areas of said target, and

automatic gain control means receiving said contrast signal as an input for supplying voltage to said photocells, the gain of said automatic gain control means being inversely proportional to the magnitude of input signal thereto.

20. Position detection apparatus in accordance with claim 19 and including means responsive to the outputs of at least two of said detector photocells for deriving a position signal whose polarity is a function of the direction of lateral deviation of said target from said reference position.

21. Position detection apparatus in accordance with claim 20 and including means responsive to the polarity of said contrast signal and of said position signal for generating an output signal whose polarity is always the same for a given direction of deviation of said target from said reference position irrespective of the relative optical contrast of different areas of said target.

22. Position detector apparatus in accordance with claim 21 wherein said target is an edge and said contrast signal deriving means is responsive to the outputs from a first pair of detector photocells which receive light from outer areas of said target spaced approximately equally on opposite sides of said reference position.

23. Position detection apparatus in accordance with claim 22 and including a second pair of detector photocells which receive light from inner areas of said target on opposite sides of said reference position disposed closer to said reference position than said outer areas, and wherein said position signal deriving means compares the algebraic average of the outputs of said first pair of photocells with the algebraic average of the outputs of said second pair of photocells to derive said position signal.

24. Position detection apparatus in accordance with claim 23 wherein said position signal deriving means includes first and second averaging circuits for deriving signals which respectively are the algebraic average of the outputs of said first pair of detector photocells and the algebraic average of the outputs of said second pair of detector photocells. 

1. Position detection apparatus for a line or edge target to be maintained in a reference position comprising, in combination, a source of illumination for said target, a plurality of detector photocells, means for transmitting to individual ones of said photocells light from discrete areas of said target spaced apart laterally of said reference position, means responsive to the outputs of at least two of said photocells for deriving a contrast signal whose polarity as a function of the relative intensity of light received from different areas of said target, means responsive to the outputs of at least two of said photocells for deriving a position signal whose polarity is a function of the direction of lateral deviation of said target from said reference position, and means responsive to the polarity of said contrast signal and of said position signal for generating an output signal of one polarity when said target deviates in one direction from said reference position and for generating an output signal of the opposite polarity when said target deviates in the opposite direction from said reference position.
 2. Position detection apparatus in accordance with claim 1 and including automatic gain control means receiving said contrast signal as an input for supplying voltage to said photocells, the gain of said automatic gain control means being inversely proportional to the magnitude of the input signal thereto.
 3. Position detection apparatus in accordance with claim 1 wherein said contrast signal deriving means includes first and second contrast differential amplifiers and a pair of similarly poled diodes each of which couples the output of one of said contrast differential amplifiers to the input of said automatic gain control means.
 4. Position detection apparatus in accordance with claim 3 wHerein the outputs of said two photocells are coupled in reverse to the inverting and non-inverting inputs of said first and second contrast differential amplifiers.
 5. Position detection apparatus in accordance with claim 1 wherein said target is an edge and said contrast signal deriving means is responsive to the outputs of a first pair of said detector photocells receiving light from areas of said target spaced apart approximately equally on opposite sides of said reference position.
 6. Position detection apparatus in accordance with claim 5 wherein a second pair of said detector photocells receives light from inner areas of said target on opposite sides of said reference position and said first pair of photocells receives light from outer areas of said target further removed from said reference position than said inner areas, and said position signal deriving means compares the algebraic average of the outputs of said first pair of photocells with the algebraic average of the outputs of said second pair of photocells.
 7. Position detection apparatus in accordance with claim 6 wherein said light transmitting means comprises optic fiber means for transmitting light reflected from said discrete areas of said target to said detector photocells.
 8. Position detection apparatus in accordance with claim 1 wherein said target is a line and said position signal deriving means is responsive to the outputs of a first pair of said detector photocells receiving light from inner areas of said target spaced approximately equal distances on opposite sides of said reference position.
 9. Position detection apparatus in accordance with claim 8 and including a second pair of said detector photocells receiving light from outer areas of said target on opposite sides of said reference position and spaced further therefrom than said inner areas, and said contrast signal deriving means compares the algebraic average of the outputs of said first pair of detector photocells with the algebraic average of the outputs of said second pair of detector photocells to derive said contrast signal.
 10. Position detection apparatus in accordance with claim 9 wherein said contrast signal deriving means includes first and second averaging circuits for deriving signals which respectively are the algebraic average of the outputs of said first pair of detector photocells and the algebraic average of the outputs of said second pair of detector photocells.
 11. Position detection apparatus in accordance with claim 10 wherein said light transmitting means comprises optic fiber means for transmitting light reflected from said discrete areas of said target to said detector photocells.
 12. Position detection apparatus in accordance with claim 10 and including automatic gain control means receiving said contrast signal as an input for supplying voltage to said detector photocells, the gain of said automatic gain control means being inversely proportional to the magnitude of the input signal thereto.
 13. Position detector apparatus in accordance with claim 12 wherein said contrast signal deriving means includes a pair of contrast differential amplifiers and a pair of similarly poled diodes coupling the outputs of said contrast differential amplifiers to the input of said automatic gain control means, and wherein the outputs of said first and second averaging circuits are connected in reverse to the inverting and non-inverting inputs of said pair of contrast differential amplifiers.
 14. Position detection apparatus in accordance with claim 13 wherein each of said detector photocells is connected in series with a resistance to form a voltage divider and said automatic gain control means generates substantially equal positive and negative voltages with respect to ground which are coupled to the ends of said voltage divider of each of said detector photocells.
 15. Position detection apparatus in accordance with claim 6 wherein said position signal deriving means includes first and second averaging ciRcuit means for deriving signals which respectively are the algebraic average of the outputs of said first pair of detector photocells and the algebraic average of the outputs of said second pair of photocells.
 16. Position detection apparatus in accordance with claim 15 and including automatic gain control means receiving said contrast signal as an input for supplying voltage to said detector photocells, the gain of said automatic gain control means being inversely proportional to the magnitude of the input signal thereto.
 17. Position detection apparatus in accordance with claim 16 wherein said contrast signal deriving means includes a pair of contrast differential amplifiers and a pair of similarly poled diodes coupling the outputs of said contrast differential amplifiers to the input of said automatic gain control means and wherein the outputs of said photocells of said first pair are coupled in reverse to the inverting and non-inverting inputs of said contrast differential amplifiers.
 18. Position detection apparatus in accordance with claim 17 wherein each of said detector photocells is of the photoresistive type and is connected in series with a resistor having a resistance approximately equal to that of said photocell to form a voltage divider and said automatic gain control means generates substantially equal positive and negative voltages with respect to ground which are coupled to the ends of said voltage divider for each of said detector photocells.
 19. Position detection apparatus for a line or edge target to be maintained in a reference position comprising, in combination, a source of illumination for said target, a plurality of detector photocells, means for transmitting to individual ones of said photocells light from discrete areas of said target spaced apart laterally of said reference position, means responsive to the outputs of at least two of said photocells for deriving a contrast signal whose magnitude is a function of the relative intensity of light received from different areas of said target, and automatic gain control means receiving said contrast signal as an input for supplying voltage to said photocells, the gain of said automatic gain control means being inversely proportional to the magnitude of input signal thereto.
 20. Position detection apparatus in accordance with claim 19 and including means responsive to the outputs of at least two of said detector photocells for deriving a position signal whose polarity is a function of the direction of lateral deviation of said target from said reference position.
 21. Position detection apparatus in accordance with claim 20 and including means responsive to the polarity of said contrast signal and of said position signal for generating an output signal whose polarity is always the same for a given direction of deviation of said target from said reference position irrespective of the relative optical contrast of different areas of said target.
 22. Position detector apparatus in accordance with claim 21 wherein said target is an edge and said contrast signal deriving means is responsive to the outputs from a first pair of detector photocells which receive light from outer areas of said target spaced approximately equally on opposite sides of said reference position.
 23. Position detection apparatus in accordance with claim 22 and including a second pair of detector photocells which receive light from inner areas of said target on opposite sides of said reference position disposed closer to said reference position than said outer areas, and wherein said position signal deriving means compares the algebraic average of the outputs of said first pair of photocells with the algebraic average of the outputs of said second pair of photocells to derive said position signal.
 24. Position detection apparatus in accordance with claim 23 wherein said position signal deriving means includes first and second averaging circuits for deriving signals which respectively Are the algebraic average of the outputs of said first pair of detector photocells and the algebraic average of the outputs of said second pair of detector photocells.
 25. Position detection apparatus in accordance with claim 24 wherein said light transmitting means comprises optic fiber means for transmitting light reflected from said discrete areas of said target to said detector photocells.
 26. Position detection apparatus in accordance with claim 21 wherein said target is a line and said position signal deriving means is responsive to the outputs of a first pair of photocells which receive light from inner areas of said target spaced approximately equally on opposite sides of said reference position.
 27. Position detection apparatus in accordance with claim 26 and including a second pair of said detector photocells receiving light from outer areas of said target on opposite sides of said reference position and spaced further from said reference position than said inner areas, and said contrast signal deriving means compares the algebraic average of the outputs of said first pair of detector photocells with the algebraic average of the outputs of said second pair of photocells to derive said contrast signal.
 28. Position detection apparatus in accordance with claim 27 wherein said contrast signal deriving means includes first and second averaging circuits for deriving signals which respectively are the algebraic average of the outputs of said first pair of detector photocells and the algebraic average of the outputs of said second pair of photocells.
 29. Position detector apparatus in accordance with claim 28 wherein said light transmitting means comprises optic fiber means for transmitting light reflected from said discrete areas of said target to said detector photocells.
 30. Position detection apparatus comprising, in combination, a source of illumination for a target to be maintained in a reference position, a first pair and a second pair of detector photocells, means for transmitting light from inner areas of said target on opposite sides of said reference position to said detector photocells respectively of said first pair and for transmitting to said photocells of said second pair respectively light from outer areas of said target on opposite sides of said reference position and further removed therefrom than said inner areas, first and second averaging circuits for deriving outputs which respectively are functions of the algebraic average of the outputs of said first pair of photocells and the algebraic average of the outputs of said second pair of photocells, means for generating a first signal whose polarity is a function of the relative magnitude of the outputs of said first and second averaging circuits, means for generating a second signal whose polarity is dependent upon the relative magnitude of the outputs of one of said pairs of photocells, and means responsive to the polarity of said first signal and of said second signal for generating an output signal of one polarity when said target deviates in one direction from said reference position and for generating an output signal of the opposite polarity when said target deviates in the opposite direction from said reference position.
 31. Position detection apparatus in accordance with claim 30 wherein said target is an edge and said second signal is a contrast signal whose polarity is dependent upon the relative magnitude of the outputs of said second pair of photocells.
 32. Position detection apparatus in accordance with claim 31 wherein said first signal is a position signal whose polarity is a function of the direction of lateral deviation of said target from said reference position.
 33. Position detection apparatus in accordance with claim 30 wherein said target is a line and said second signal is a position signal whose polarity is dependent upon the relative magnitudes of the outputs of said first pair of photocells.
 34. Position detection apparAtus comprising, in combination, a source of illumination for a target to be maintained in a reference position, a first pair and a second pair of detector photocells, means for transmitting light from inner areas of said target on opposite sides of said reference position respectively to said photocells of said first pair and for transmitting light to said photocells of said second pair from outer areas of said target on opposite sides of said reference position and further removed therefrom than said inner areas, first and second averaging circuits for deriving signals which respectively are the algebraic average of the outputs of said first pair of photocells and the algebraic average of the outputs of said second pair of photocells, a pair of contrast differential amplifiers; a pair of differential preamplifiers adapted to be switched between an off position wherein their impedance is high and an active position, means responsive to the outputs of said contrast differential amplifiers for controlling the condition of said differential preamplifiers, and an output amplifier having its input coupled to the outputs of both of said differential preamplifiers, the outputs of said photocells of said second pair being connected in reverse to the inverting and non-inverting inputs of said contrast differential amplifiers and the outputs of said first and second averaging circuits being connected in reverse to the inverting and non-inverting inputs of said differential preamplifiers.
 35. Position detection apparatus in accordance with claim 34 and including switching means for disconnecting said second pair of photocells from said contrast differential amplifiers and for disconnecting the outputs of said first and second averaging circuits from said differential preamplifiers and for connecting them in reverse to the inverting and non-inverting inputs of said contrast differential amplifiers and for also connecting the outputs of said photocells of said first pair in reverse to the inverting and non-inverting inputs of said differential preamplifiers.
 37. Position detection apparatus in accordance with claim 36 wherein each of said detector photocells is connected in series with a resistance to form a voltage divider and the supply voltage from said automatic gain control means is coupled across said voltage divider.
 38. Position detection apparatus in accordance with claim 36 wherein said automatic gain control means generates substantially equal positive and negative voltages with respect to ground which are coupled to the ends of said voltage divider for each of said detector photocells.
 39. Position detection apparatus comprising, in combination, a source of illumination for an edge target to be maintained in a reference position, a first pair and a second pair of detector photocells, means for transmitting light from inner areas of said target on opposite sides of said reference position respectively to said photocells of said first pair and for transmitting light to said photocells of said second pair respectively from outer areas of said target on opposite sides of said reference position and further removed therefrom than said inner areas, means for generating a contrast signal whose polarity is dependent upon the relative magnitude of the outputs of said photocells of said second pair, first and second averaging circuit means for deriving outputs which respectively are functions of the algebraic average of the outputs of said first pair of photocells and the algebraic average of the outputs of said second pair of photocells, means for generating a position signal whose polarity is dependent upon the relative magnitudes of the outputs from said first and second averaging circuit means, and means responsive to the polarity of said contrast signal and of said position signal for generating an output signal of one polarity when said edge target deviates in one direction from said reference position and for generating an output signal of the opposite polarity when said edge target deviates in the opposite direction from said reference position.
 40. Position detection apparatus comprising, in combination, a source of illumination for a line target to be maintained in a reference position, a first pair and a second pair of detector photocells, means for transmitting light from inner areas of said target on opposite sides of said reference position respectively to said photocells of said first pair and for transmitting light to said photocells of said second pair respectively from outer areas of said target on opposite sides of said reference position and further removed therefrom in a lateral direction than said inner areas, means for generating a position signal whose polarity is dependent upon the relative magnitude of the outputs from said photocells of said first pair, first and second averaging circuit means for deriving outputs which respectively are the algebraic average of the outputs of said photocells of said first pair and the algebraic average of the outputs of the photocells of said second pair, means for deriving a contrast signal whose polarity is dependent upon the relative magnitude of the outputs from said first and second averaging circuit means, and means responsive to the polarity of said contrast signal and of said position signal for generating an output signal of one polarity when said line target deviates in one direction from said reference position and for generating an output signal of the opposite polarity when said line target deviates in the opposite direction from said reference positions.
 41. Position detection apparatus comprising, in combination, a source of illumination for an edge target to be maintained in a reference position, first second, third and fourth detector photocells, means for transmitting light from inner areas of said target on opposite sides of said reference position to said first and second photocells respectively and for transmitting light to said third and fourth photocells respectively from areas of said target on opposite sides of said reference position and further removed therefrom in a lateral direction than said inner areas, means for generating a contrast signal whose polarity is dependent upon the relative magnitudes of the outputs from said third and fourth detector photocells, first and second averaging circuits for deriving output signals which respectively are functions of the algebraic average of the outputs of said third and fourth photocells and the algebraic average of the outputs of said first and second photocells, first and second differential preamplifiers each of which is adapted to be switched between an off condition wherein its impedance is high and an active condition, the outputs of said first and second averaging circuit being connected in reverse to the inverting and non-inverting inputs of said first and second differential preamplifiers, means responsive to the polarity of said contrast signal for controlling the condition of said first and second differential preamplifiers, and an output amplifier having its input coupled to the outputs of both said first and said second differential preamplifiers.
 42. Position detection apparatus comprising, in combination, a source of illumination for a target to be maintained in a reference position, a first pair and a second pair of detector photocells, means for transmitting light from inner areas of said target on oppositE sides of said reference position respectively to said photocells of said first pair and for transmitting to said photocells of said second pair respectively light from outer areas of said target on opposite sides of said reference position and further removed therefrom in a lateral direction than said inner areas, first and second averaging circuits for deriving signals which respectively are functions of the algebraic average of the outputs of said first pair of photocells and the algebraic average of the outputs of said second pair of photocells, a pair of contrast differential amplifiers, switching means having a first position wherein the outputs of said second pairs of photocells are coupled in reverse to the inverting and non-inverting inputs of said pair of contrast differential amplifiers and a second position wherein the outputs of said first and second averaging circuits are coupled in reverse to said inverting and non-inverting inputs, a pair of differential preamplifiers adapted to be switched between an off position wherein their impedance is high and an active position, said switching means in said first position coupling the outputs of said first and second averaging circuits in reverse to the inverting and non-inverting inputs of said pair of differential preamplifiers and in said second position coupling the outputs of said first pair of photocells in reverse to said inverting and non-inverting inputs, means responsive to the outputs of said contrast differential amplifiers for controlling the condition of said differential preamplifiers, and an output amplifier having its input coupled to the outputs of both of said differential preamplifiers.
 43. Position detection apparatus in accordance with claim 42 and including automatic gain control means for supplying voltage to said detector photocells and having a gain which is inversely proportional to the magnitude of input signal thereto, and a pair of similarly poled diodes coupling the outputs of said contrast differential amplifiers to the input of said automatic gain control means.
 46. Position detection apparatus in accordance with claim 34 and including automatic gain control means for supplying voltage to said detector photocells and having a gain inversely proportional to the magnitude of the input signal thereto, and including a pair of similarly poled diodes each of which couples the output of one of said contrast differential amplifiers to the input of said automatic gain control means. 